1. Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus including the optical scanning device, and an optical scanning method.
2. Description of the Related Art
Electrophotographic image forming apparatuses used in laser printers, laser plotters, digital copiers, plain-paper facsimiles, and multifunction peripherals have recently become increasingly faster and capable of forming color images. Image forming apparatuses of a tandem configuration provided with a plurality of (typically, four) photosensitive elements which are image carriers have come into wide use. In such a tandem image forming apparatus, for example, four photosensitive elements are arranged side by side along a conveying belt (or an intermediate transfer belt on which toner images are temporarily carried) that conveys a recording material. The tandem image forming apparatus forms a color image as follows. Electric chargers electrostatically charge the photosensitive elements. Thereafter, writing units form latent image on the photosensitive elements. Developing units develop the latent images on the photosensitive elements with respective developing materials (yellow, magenta, cyan, and black toners, for example). These developed images of the respective colors are sequentially transferred onto the recording material (or the intermediate transfer belt) conveyed by the conveying belt to be overlaid on one another, thereby forming a color image.
As another than the tandem configuration, what is referred to as a one-drum-intermediate-transfer configuration is adopted by some image forming apparatuses. This type of image forming apparatus develops toner images of respective colors one by one on a single photosensitive element, transfers the toner images onto an intermediate transfer member one by one to overlay the images on one another to form a color image on the intermediate transfer member, and thereafter transfers the images onto a recording material at a time. This configuration requires rotating the photosensitive element four times for each sheet of image formation. Accordingly, this configuration is inferior in productivity in color image formation to the tandem configuration.
As described above, tandem image forming apparatuses can form color images at higher speeds than the one-drum-intermediate-transfer image forming apparatuses and therefore can increase productivity in color image formation. However, in the tandem image forming apparatus, an optical scanning device is used as a writing unit that performs optical writing onto the plurality of photosensitive elements. Accordingly, the optical scanning device is undesirably required to include a plurality of light sources (for instance, an image forming apparatus including four photosensitive elements generally requires four light sources). As a result, an increase in the number of components, misregistration due to differences between wavelengths of the plurality of light sources, an increase in production cost, and the like problems arise.
Some type of tandem image forming apparatuses includes an optical scanning device that deflects light flux, or light beams, emitted from a plurality of light source devices associated with a plurality of photosensitive elements using a single deflector so that exposure of the photosensitive elements is performed simultaneously using a plurality of scanning-image-forming optical systems associated with the photosensitive elements, thereby forming latent images.
When such an optical scanning device is employed, a configuration what is referred to as an oblique-incidence optical system is typically employed for cost reduction. For example, a technique that causes light beams to be incident on a deflection-reflection surface of a deflector obliquely with respect to the sub-scanning direction is disclosed in Japanese Patent Application Laid-open No. 2003-005114. In this incidence optical system, each of the plurality of light beams is deflected and reflected by the deflection-reflection surface. Thereafter, the plurality of light beams are separated and guided onto corresponding scanned surfaces (photosensitive element surfaces) by a turning mirror or the like. Angles (angles of incidence on the optical deflector) of the plurality of light beams with respect to the sub-scanning direction are set to angles that allow the turning mirror or optical elements associated with the plurality of light beams to separate the light beams from one another.
In other words, employing the oblique-incidence optical system makes it possible to achieve intervals, at which the turning mirror can separate the light beams from one another, between the light beams in the sub-scanning direction without upsizing an optical deflector. This leads to a less-expensive optical scanning device. The upsizing can result from, for example, increasing the thickness of a polygon mirror of the optical deflector or adopting a polygon mirror that is multi-layered with respect to the sub-scanning direction of the light beams. When a polygon mirror is used as the optical deflector, large energy that would otherwise be required for high-speed spinning becomes unnecessary, and wind noise caused by the high-speed spinning can be reduced.
In an image forming apparatus that employs the oblique-incidence optical system, a flat glass is typically arranged on optical paths of the plurality of light beams at a position near the optical deflector for the sake of reducing wind noise caused by high-speed spinning of the optical deflector, reducing transfer of heat that is generated at a motion section of the optical deflector, preventing mirror surfaces of the optical deflector from being smudged, and the like.
Put another way, the plurality of light beams deflected and reflected by the optical deflector enter the scanning optical system via the flat glass. The flat glass is arranged obliquely with respect to the sub-scanning direction with reference to the plane of the normal of the deflection-reflection surface of the optical deflector. Accordingly, when the flat glass is tilted in the sub-scanning direction, some of the light beams does not return to the optical deflector, but the other of the light beams returns to be incident on the optical deflector again. In this case, there arises a problem that the light beam incident on the optical deflector again (referred to as stray light) reaches a scanned surface (photosensitive element surface), thereby degrading image quality.
Meanwhile, stray light is, contrasted with a light beam (writing light) that scans a scanned surface, unnecessary light resulting from reflection from an optical element or the like on an optical path of the light beam. For example, a light beam resulting from reflection from the flat glass is stray light.
A technique of arranging a flat glass parallel to a deflection-reflection surface of an optical deflector in an oblique-incidence optical system is disclosed in Japanese Patent Application Laid-open No. H08-334719. This technique is effective as a countermeasure against stray light in an optical system including only one scanned surface. However, this technique cannot prevent generation of stray light nor reduce an amount of stray light reaching a scanned surface when this technique is applied to an optical scanning device (or an image forming apparatus) that includes scanned surfaces to be scanned with a plurality of light beams and causes the plurality of scanning light beams to be incident on a same deflection-reflection surface of an optical deflector obliquely from opposite sides with reference to the plane of the normal of the deflection-reflection surface.
A technique related to a flat glass arranged upstream of incidence of a light beam on an optical deflector is disclosed in Japanese Patent Application Laid-open No. H08-334720. This technique causes the flat glass to tilt in a direction that causes a light beam that is reflected by the flat glass before incidence on the optical deflector and a light beam that is reflected from the optical deflector to deviate from each other. However, because the plurality of light beams are obliquely incident on a same deflection-reflection surface of the optical deflector from opposite sides with reference to the plane of the normal of the deflection-reflection surface, this technique cannot prevent generation of stray light from every light beam nor reduce an amount of stray light reaching a scanned surface.
Therefore, there is a need for an optical scanning device and an optical scanning method capable of preventing generation of stray light or reducing an amount of stray light reaching a scanned surface and an image forming apparatus including the optical scanning device.